M.C. Lanca

The fractal analysis of water trees: an estimate of the fractal dimension

Water trees result from ac electrical aging of the polymeric insulation of medium and HV power cables in a humid or wet environment. As suggested by their name, they arise from penetration of water in the polymer. Visual observation with the help of an optical microscope shows tree (bush) type structures. This suggests that water trees might be fractal objects. Calculation of the fractal dimension from experimental samples may confirm the fractal characteristics and also give information on the damage caused to the polymer. In this work images of water trees taken under the optical microscope, dyed by methylene blue and etched for scanning electron microscopy (SEM), were studied in order to estimate the fractal dimension using a box-counting algorithm. The photographs, made using an optical microscope (scale of 100 /spl mu/m), of the dyed samples were obtained from laboratory-aged low-density polyethylene (LDPE) specimens using accelerated techniques. Different field amplitude and frequency and also time of aging were used and the dimension values were compared. SEM images resulting from aged cross-linked polyethylene (XLPE) cables revealed a structure at a different scale (/spl sim/3 /spl mu/m). Each photograph was analyzed to compare regions with and without water trees.

Space charge studies in LDPE using combined isothermal and nonisothermal current measurements

Using a recently developed procedure combining isothermal and nonisothermal current measurements space charge trapping and transport in LDPE was successfully studied. Unaged, thermally and electrically aged samples were investigated. The samples were conditioned before each measurement in order to obtain reproducible results. In the nonisothermal measurements appeared a broad peak (40/spl deg/C to 50/spl deg/C) that was possible to decompose into two or three peaks (35, 45 and 65/spl deg/C). At even higher temperature another peak was sometimes present (85/spl deg/C) depending on the prior sample conditioning. The space charge is trapped near the surface in deep traps (maximum depth of /spl ap/15 /spl mu/m). Relaxation times, mobilities and activation energies have been calculated for different charging/discharging conditions. For unaged samples the reproducibility of the results was poor while for the aged polyethylene it was quite good, meaning that aging helps conditioning. In the electrically aged LDPE there is a decrease of conductivity and the broad peak of the nonisothermal spectra shows a slight shift towards higher temperatures when compared with the data found in the thermally aged polymer.

Studies of space charge in electrically aged low density polyethylene

Space charge in electrically aged LDPE was studied using a recently developed technique combining isothermal charging and discharging with non-isothermal measurements. Samples were aged in a NaCl aqueous solution at 40/spl deg/C for 1500 h under an AC field of 6 MV/m (50 Hz). The samples were then isothermally DC charged and discharged (both currents recorded). Next a non-isothermal experiment with constant heating rate was performed. Finally, the sample was kept at the highest temperature and the final isothermal discharge current registered. The last step has to be carried on for a long time to ensure an almost complete discharge of the remnant charge so that results become reproducible and possible to analyze. Selective charging (careful choice of the field, temperature and the ratio of charging/discharging times) revealed the presence of different trapping sites. From the analysis of the isothermal and non-isothermal data the relaxation times and activation energies could be obtained.

Comparative study of space charge in the polymeric insulation of power cables using PEA, isothermal and non-isothermal currents measurements

An understanding of space charge build-up in the polymeric insulation of power cables is important in determining how aging occurs and progresses and, also in predicting cable lifetime. In this investigation electric-field induced space charge in peelings from XLPE (cross-linked polyethylene) cables was measured using two different methods: the pulsed electro-acoustic technique (PEA) and the combined procedure of isothermal and non-isothermal charging/discharging currents (FTSDC). These two methods allow the study of space charge in highly insulating materials. Also, since electric fields of different orders of magnitude are applied to the sample in the two methods, it is possible to analyze different characteristics of the space charge traps. Prior to the measurements the samples were subjected to conditioning to remove volatiles. Cable peelings from various brands aged under different conditions (including field aged and thermally aged samples) were studied as received from the manufacturers. Some of the samples have undergone further ageing in AC electric field (50 Hz) for 1000 h to see the influence of further ageing on space charge build-up. The results for the different types of samples are compared in an attempt to correlate different ageing parameters.

Comparative study of dielectric relaxation spectra of electrically and thermally aged low density polyethylene

Low-density polyethylene (LDPE) films were thermally aged in a sodium chloride aqueous solution at constant temperature (thermal aging). Some of the samples were simultaneously immersed in solution and subjected to an electric AC field (electrical aging). The dielectric relaxation spectra at 30/spl deg/C in the range of 10/sup -5/ Hz to 10/sup 5/ Hz were obtained for unaged and aged samples. For the low frequency (LF) region (10/sup -5/ Hz to 10/sup -1/ Hz) the time domain technique was used. A lock-in amplifier was used for the 10/sup -1/ Hz to 10/sup 1/ Hz medium frequency (MF) region. While for the high frequency (HF), 10/sup -1/ Hz to 10/sup 5/ Hz, RLC bridge measurements were performed. The main differences can be seen between electrically, thermally aged and unaged LDPE in the HF and LF regions. The LF peak is a broad peak related to localized space charge injection driven by the electric field. For electrically aged samples this peak increases in an earlier stage of electrical aging, decreasing afterwards. While in thermally aged samples the peak amplitude always increases with aging time. Finally the HF shows the beginning of a peak due to the /spl gamma/ and /spl beta/ transitions. This peak decreases with aging disappearing for the most aged samples.

Space charge and dipolar charge contribution at polar polymers polarization

The thermally stimulated discharge current, the final thermally stimulated discharge current, DC conductivity and the final thermally stimulated discharge current with partially blocking electrode measures were used to analyze electrical behavior of Nylon 11. The objective was to discriminate between dipole related effects and space charge related effects. The space charge effects are dominant in the temperature range from room temperature to 170 °C. By using a Teflon-FEP partially blocking electrode, the space charge injected in the sample is diminished and the effects related to dipole movement can be observed. Beside the two known relaxations for Nylon 11, one associated with the glass transition around 60 °C and a second one associated with a molecular motion in the rigid-amorphous phase at 96 °C, a weak relaxation was observed around 168 °C. The peak around 96 °C is quite broad been composed of two narrow peaks. The final thermally stimulated discharge current method allows a better selection of the experimental conditions for sample charging (polarization) to have only a partial overlap between the nearby peaks. The peaks maximum current and temperature are dependent on the ratio between the charging and discharging time and temperature given a possibility to discriminate between dipolar and space charge effects. A pyroelectric current changes sign around 140 °C indicating that the amidegroup dipoles are frozen in opposite directions when the sample temperature is below 140 °C (amorphous and rigid-amorphous phase) or above (crystalline phase). The conductivity is controlled by the competition between n(E,T) and μ(E,T) indicating a space charge controlled conductivity mechanism.

Identification of an apparent peak by use of the final thermally stimulated discharge current technique

The final thermally stimulated discharge current (FTSDC) technique can be used to analyze charge trapping and transport in insulating materials. The experimental conditions can be selected so that the FTSDC is mainly determined by the space charge detrapping. Measurements of the FTSDC in a wide temperature range including the local (secondary) (/spl beta/ relaxation and the non-local (primary) /spl alpha/ relaxation, for different polymers, demonstrate the existence of an apparent peak. The shift of peak temperature T/sub m/ with respect to the charging temperature T/sub p/ is analyzed. The interval T/sub m/ - T/sub p/ decreases from about 25 K to zero, as T/sub p/ approaches the glass transition T/sub g/. T/sub m/ - T/sub p/ is lower for materials of lower conductivity. The peak width at the half maximum intensity decreases as T/sub p/ increases and the thermal apparent activation energy increases. The variations are not monotonous revealing the temperature range where the molecular motion is stronger and consequently the charge trapping and detrapping processes are affected by the strong thermal motion.

Discrimination between space charge and dipolar contributions in ferroelectric polymers

The final thermally stimulated discharge current method allows a better selection of the experimental conditions for sample polarization. By decreasing the ratio between the charging time and the discharging time, the apparent peak is of the same order of magnitude as the genuine peaks and there is only a partial overlap between then. Two peaks have been identified for polyamide 11, one associated with the glass transition around 60 °C and the second associated with the Curie transition around 96 °C.

Influence of polarization on the bioactivity of nanopowders of hydroxyapatite

Polarization can accelerate the bioactivity of HAp immersed in an SBF solution. However a careful choice of polarization conditions has to be made. One relevant issue is to have long relaxation times of the polarization and this is related to the amount of water in Hap, which was to be kept as low as possible during poling. It was shown that the precipitation mechanism needs negative charges on the Hap surface. Under positive charge the mechanism is inhibited.

The study of the molecular movements in the range of glass transition by the final thermally stimulated discharge current technique

The electrical methods used to study the molecular movements are based on the movement of the dipoles under DC or AC electric field. We have proposed recently a combined measuring protocol to analyze charge injection/extraction, transport, trapping and de-trapping in polar or non-polar dielectric materials. The method is used here to analyze the molecular movements in polyimide in the temperature range from 293 to 572 K. A strong relaxation was observed around 402 K and a very weak relaxation around 345 K. This is the β relaxation which is quite complex. As concern the behavior at high temperatures, above the β relaxation, a high peak was observed that shifts continuously to higher temperatures as the charging temperature and/or the charging field increase. The maximum current of the peak increases and the temperature corresponding to the maximum current increases as the charging temperature and/or the charging field increase, given a direct observation of the so called cross-over effect related to current decay for sample charged at high fields and/or high temperatures.